Maintaining a healthy weight is important for young children because childhood obesity is predictive of adult obesity and related chronic illness. Intake of sugar-sweetened beverages (SSBs: sodas, flavored milks, fruit-flavored drinks, and other drinks with added sugar) is a major contributor to obesity. This is particularly true for low income children who are more likely to drink SSBs and to be obese. Children spend substantial time in schools where they consume up to 50% of daily calories, including those from SSBs. Consequently, many obesity prevention efforts have targeted reductions in SSB intake in schools. Recently, scientific authorities have also recommended that schools improve the availability of potable and free drinking water - a healthy alternative to SSBs - as a low-cost and feasible obesity prevention strategy. Emerging policies also mandate water access in schools; but implementation is poor. Although the 2010 Healthy, Hunger-Free Kids Act requires schools participating in federal meal programs to provide free potable water where meals are served, 25% of US schools still fail to do so. And even in schools that offer free water, drinking fountains may be avoided due to concerns about cleanliness or sub-standard water quality. While promotion of drinking water intake in schools is a plausible obesity prevention strategy, no large studies have systematically examined how this tactic can change children's overall dietary patterns and obesity rates in schools that offer SSBs and juices. Our central hypothesis is that in elementary schools, increased access to fresh water and rigorous promotion of its consumption will reduce student intake of caloric beverages, thereby leading to lower rates of obesity. To test this hypothesis, we will conduct a cluster-randomized trial in 26 low-income elementary schools in the San Francisco Bay Area, in which 13 schools will receive a water promotion intervention and 13 schools will serve as controls. The intervention, based on Social Cognitive Theory and the PRECEDE-PROCEED Model and cultivated in prior developmental studies by our team, promotes water consumption by: 1) installing lead-free water stations in cafeterias, physical activity spaces and high-traffic common areas, 2) providing cups and reusable water bottles for students, and 3) conducting a 6-month health education campaign that includes a kick-off play, class lessons, family homework activities, signage, and rewards. From baseline to 9 and 15 months after the start of the intervention, we will measure differences in: 1) water intake (observations and measurements of water taken from water sources) 2) total caloric intake from foods and beverages (24-hour food and beverage diaries) and 3) overweight/obesity prevalence between students in intervention and control schools. If the proposed school water intervention is effective, we can provide school officials with a feasible and low-cost obesity prevention tool. US schools will soon be federally mandated to reevaluate their wellness policies in order to implement new food and beverage regulations. This study presents a timely opportunity to provide leaders with an evidence-based strategy for improving student nutrition and health.